For industrial pressure-sensitive adhesive (PSA) tape applications it is very common to employ double-sided PSA tapes in order to join two materials to one another. A distinction is made here, depending on type, between single-layer double-sided self-adhesive tapes and multilayer double-sided self-adhesive tapes.
Single-layer double-sided self-adhesive tapes, known as transfer tapes, are constructed such that the PSA layer may indeed include fibers or fillers but contains no carrier and is lined only with corresponding release materials, such as siliconized release papers or release films. The term “adhesive transfer tape” derives from the transfer of the PSA attribute to a different material (profiles, foams, etc). Transfer tapes may be lined with release materials on one side or both sides. Often use is made of release papers or release films with different degrees of siliconization on either side, so that the transfer tape can be wound readily into a roll and then also applied readily. Such adhesive transfer tapes are frequently used in order to provide any of a very wide variety of substrates with pressure-sensitive adhesion. This is accomplished, for example, by laminating the transfer tape onto the substrate. In that case the release paper remains as a liner to the PSA layer in the product.
Transfer tapes are frequently produced from solution. These tapes are in general relatively thin, since at higher coatweights the layers may become blistery. It is known that conventional coating techniques for the production of PSA layers with a thickness of more than 100 μm, but in particular above 200 μm, are problematic. For instance, the drying of solvent-borne or water-based thick PSA layers is accompanied by formation of solvent blisters, thereby impairing the optical and the technical adhesive properties of the dried PSA layer. It is therefore necessary to reduce the coating rate when the amount of PSA to be applied is increased, which makes products produced in this way unprofitable.
Particularly for the production of relatively thick transfer tapes, solvent-free methods are of advantage. The technological operation of producing and coating PSAs is undergoing continual onward development. Within the industry, hotmelt methods with solvent-free coating technology are of growing importance for the production of PSAs. This development is being forced onward by evermore stringent environmental strictures and by rising prices for solvents. One aim of the development is therefore to eliminate solvents as far as possible from the manufacturing operation for PSA tapes. Furthermore, it avoids the problem of formation of blisters and other irregularities in the coatings. A particularly advantageous feature of the hotmelt technology is the facility for coating at very high speeds.
As the PSA of a transfer tape it is possible in principle to use any material possessing pressure-sensitive adhesion. Use is often made of compositions based on natural rubber, synthetic rubber, polyurethane or polyacrylate. With regard to the hotmelt technology, therefore, compositions based on styrene block copolymers are popular, on account of their rapid and efficient coatability. Coatings based on styrene block copolymers do not exhibit any profile of crosslinking through the layer. A disadvantage of PSA coatings based on styrene block copolymers, such as SIS, SBS, SEBS or SEPS, is their low UV stability and aging resistance. A particularly disadvantageous feature of transfer tapes based on styrene block copolymers is their very low heat resistance. Above about 70-90° C., depending on formulation, the compositions soften, and the bonds fail. The acrylate block copolymer-based compositions being newly developed at present are significantly more resistant to aging. Moreover, they allow water-clear, transparent, pressure-sensitively adhesive coatings. However, since they are crosslinked only physically via styrene or methyl methacrylate domains, these systems also soften as soon as the application temperature is above the softening temperature of the domains. Both systems are unsuitable for applications at relatively high temperatures.
For high-end industrial applications, preference is given to crosslinked polyacrylates, on account of their water-clear transparency and weathering stability. Moreover, because of their saturated polymer backbone, polyacrylates are very aging-resistant, insensitive to alteration by irradiation with UV or sunlight, stable to ozonolysis, and, depending on comonomer composition, inherently pressure-sensitively adhesive. Blending with migratable constituents such as resins and plasticizers is often unnecessary. Crosslinked polyacrylates are highly resistant to a shearing load, even at high temperatures.
In the prior art, solvent-free, acrylate-based transfer tapes are frequently produced by methods involving radiation chemistry. For example, transfer tapes can be produced by UV prepolymerization or UV crosslinking of hotmlet PSAs. All of these products exhibit a gradient of crosslinking through the layer.
DE 43 03 183 A1 describes a method of producing thick PSA layers, especially for producing high-performance self-adhesive articles. In said process a mixture of starting monomers which is to be polymerized by means of UV radiation is mixed, and thickened in the process, with a solvent-free, saturated photopolymerizable polymer, and then this mixture is applied to a dehesively treated carrier and exposed to UV radiation. A disadvantage is the use of copolymerized or added photoinitiators, since the layers may undergo yellowing and, in the event of UV exposure prior to use, may suffer marked changes in the technical adhesive properties. In that case it is necessary to go to considerable effort and expense—by means, for example, of UV-impervious packaging—to ensure that the customer obtains a uniformly high bonding performance. Moreover, in the event of bonding on UV-transparent substrates, such as on window glass or transparent plastic surfaces, for example, there is a risk that layers containing photoinitiator will undergo aftercrosslinking. This does result initially in an increase in bond strength, but further crosslinking causes the layers to become paintlike and undergo embrittlement. Sooner or later, this leads to the failure of the bond, particularly under a shearing load.
A disadvantage in the case of all radiation-crosslinked layers, and especially in the case of UV-crosslinked layers, is a more or less strongly pronounced profile of crosslinking through the layer. Toward the irradiation source, the UV-crosslinked layer is always more strongly crosslinked than on the side opposite the UV radiation source. The degree of the crosslinking profile is dependent for example on the layer thickness, on the wavelength of the photoinitiator that is used, and also on the wavelength of the radiation emitted by the UV radiation source.
DE 198 46 902 A1 and DE 101 63 545 A1 propose using EBC (electron beam) irradiation or UV irradiation from both sides in order to reduce the resulting crosslinking profile and to provide virtually homogeneous crosslinking of thick UV-crosslinkable acrylate PSA layers in particular. However, even the layers produced in this way have a crosslinking profile, and, moreover, the process is very costly and inconvenient.
EBC-crosslinked layers always exhibit a profile of crosslinking in accordance with the layer thickness and the material. With EBC-crosslinked layers as well it is impossible to set the crosslinking exactly. Nevertheless, EBC crosslinking proceeds without added photoinitiators, thereby removing some, although not all, of the disadvantages associated with the UV-irradiated layers. Depending on the accelerator voltage and on the thickness of the material to be irradiated, it is possible to vary the thickness of the irradiated layer. Layers above about 500 μm in thickness, particularly if filled with inorganic fillers such as glass balls, for example, can no longer be economically irradiated, and so there is an upper limit on the PSA layer thicknesses of PSA tapes.
For some time, UV-crosslinkable hotmelt PSAs have been available commercially under the trade name acResin®. On account of their relatively low weight-average molecular weight (Mw approximately 200 000-300 000 g/mol), these compositions lend themselves to very effective coating and subsequent crosslinking by UV irradiation. Disadvantages, however, are again the inhomogeneity of the crosslinking through a dose profile, and also a low level of efficiency in the case of resin-modified acrylate compositions, and the limitation on layer thickness to well below 100 μm, which rules out use for substantial areas of industrial adhesive tapes.
In accordance with prior art, transfer tapes are also produced by an operation of two-stage UV polymerization, but one which has decisive disadvantages. In the first step of that process a mixture based on acrylate monomers is prepolymerized to a conversion of approximately 10%-20% by UV irradiation in a reactor in the presence of a photoinitiator. Alternatively, this “acrylic syrup” can also be obtained by thermally initiated free radical polymerization. In the second step this acrylic syrup, optionally after further photoinitiators, fillers, hollow glass balls, and crosslinkers have been added, is coated between antiadhesively coated UV-transparent films, and is polymerized to a higher degree of conversion on the web, by means of repeated UV irradiation, and in the course of this polymerization it is crosslinked.
The production of “relatively thick” viscoelastic layers in particular must in many cases be carried out in the absence of oxygen. In that case the composition is protected by a lining of film material, and UV initiation takes place through the films. PE and PP films which are sometimes used for this purpose deform under the conditions of crosslinking reaction (in the case of UV-initiated polymerization, heat of reaction is liberated, and can cause deformation of non-temperature-resistant film) and are therefore poorly suited. UV-transparent films such as PET are more thermally stable; in this case, however, it is necessary to add to the composition a photoinitiator which reacts to longwave radiation, in order for the reaction to take place. As a consequence of this, these layers have a tendency to undergo aftercrosslinking under UV light or sunlight. This process negates the advantage specific to the polyacrylate as a material. A further disadvantage is that fillers not transparent to UV cannot be used. Moreover, as a result of the process, there remains a high residual monomer fraction in these products. Possible reduction of residual monomer through a reduction in coating speed or through intensive subsequent drying is not very economic. The maximum achievable layer thickness is very heavily dependent on the wavelength of the photoinitiator used. Layers can be produced of up to about 1 mm, albeit with the disadvantages specified above. Layers any thicker than this are virtually impossible to obtain.
Transfer Tapes which have been Produced by Two-Stage UV Polymerization Also Exhibit a Profile of Crosslinking Through the Layer.
A disadvantage of transfer tapes which exhibit a profile of crosslinking through the layer is their inadequate capacity for distributing stresses in a uniform way. One side is always either overcrosslinked or undercrosslinked. An exact balance can never be struck between adhesive and cohesive properties for the entire layer, but only for a small section.
It is an object of the invention, accordingly, to overcome the disadvantages of the prior art and to provide transfer tapes which do not exhibit any profile of crosslinking through the layer.